Opportunities and challenges of entomopathogenic nematodes as biocontrol agents in their tripartite interactions

Background The complex including entomopathogenic nematodes (EPNs) of the genera Steinernema and Heterorhabditis and their mutualistic partner, i.e., Xenorhabdus and Photorhabdus bacteria, respectively possesses many attributes of ideal biological control agents against numerous insect pests as a third partner. Despite authenic opportunities for their practical use as biocontrol agents globally, they are challenged by major impediments especially their cost and reliability. Main body This review article presents major attributes of EPNs to familiarize growers and stakeholders with their careful application. As relatively high EPN costs and frequently low efficacy are still hindering them from reaching broader biopesticide markets, this is to review the latest findings on EPN strain/species enhancement, improvement of production, formulation and application technology, and achieving biological control of insects from the standpoint of facing these challenges. The conditions and practices that affected the use of EPNs for integrated pest management (IPM) are identified. Besides, efforts have been made to address such practices in various ways that grasp their effective approaches, identify research priority areas, and allow refined techniques. Additionally, sampling factors responsible for obtaining more EPN isolates with differential pathogenicity and better adaptation to control specific pest(s) are discussed. Conclusion Specific improvements of EPN production, formulation, and application technology are reviewed which may help in their broader use. Other diverse factors that optimize EPNs to constitute a cost-effective, value-added approach to IPM are also demonstrated.

Fungi against insects and contrariwise as biological control models

Background Biological control of insects is the current goal of modern researches to avoid using the harmful chemicals. Some fungi are capable of infecting and killing insects and, hence, are commonly known as entomopathogenic fungi (EPF). On the other hand, some insects can kill harmful fungal strains using their products such as peptides. Hence, the aim of this review article is to highlight the use of EPF as biocontrol tools against each other. Results EPF are generally characterized by having a wide range of hosts which made them the perfect candidate for biological control missions. They are existing in abundance in the environment and involved in plenty of environmental interactions. They have prestigious enzymatic machinery and toxins that contribute as killing tools. Moreover, after penetrating the insect, the expanded vegetative growth of hyphal bodies enabling the invasion of the fungi throughout the entire tissues of host insect cause physic, histolytic, and pathologic changes ultimately leading to the death of the host insect. On the other hand, some insects can kill harmful fungal strains using their secreted products such as peptides. Conclusion In this review, the use of fungi and insects as biological control agents against each other was described. Furthermore, the history of using EPF for this purpose, their killing mechanism, host range, and the factors affecting EPF virulence were highlighted. Moreover, the role of insect’s immunology and some insect’s products as antifungal agents was presented focusing on peptides with biological activities against fungi. Finally, future prospects concerning the use of insects and fungi in biological control process were discussed.

Potential for Bacillus thuringiensis and Other Bacterial Toxins as Biological Control Agents to Combat Dipteran Pests of Medical and Agronomic Importance

The control of dipteran pests is highly relevant to humans due to their involvement in the transmission of serious diseases including malaria, dengue fever, Chikungunya, yellow fever, zika, and filariasis; as well as their agronomic impact on numerous crops. Many bacteria are able to produce proteins that are active against insect species. These bacteria include Bacillus thuringiensis, the most widely-studied pesticidal bacterium, which synthesizes proteins that accumulate in crystals with insecticidal properties and which has been widely used in the biological control of insects from different orders, including Lepidoptera, Coleoptera, and Diptera. In this review, we summarize all the bacterial proteins, from B. thuringiensis and other entomopathogenic bacteria, which have described insecticidal activity against dipteran pests, including species of medical and agronomic importance.

In vivo production of entomopathogenic nematodes suppressed by asymptomatic bacterial contaminants

Summary Entomopathogenic nematodes (EPN) are excellent biological control agents possessing recycling ability as one of their major attributes. We report the presence of asymptomatic bacteria that can lead to disrupted or low progeny production in Heterorhabditis indica. In a one-to-one in vitro competitive bioassay with contaminants associated with H. indica cuticle, there was a significant suppression in the growth of Sphingomonas koreensis when stressed with the nematode symbiont Photorhabdus luminescens; however, P. luminescens was suppressed when sandwiched between Ochrobactrum anthropi. Bacillus bombysepticus associated with laboratory-reared Galleria when stressed by P. luminescens was significantly suppressed, but not so in the reverse assay. Both O. anthropi and B. bombysepticus were found to be insecticidal to Galleria larvae when fed orally. Tripartite interactive studies on the growth and multiplication of H. indica-P. luminescens symbionts in Galleria larvae, predisposed to S. koreensis, revealed no significant difference initially in the hermaphrodite formation, but subsequently there was a significant decline in the formation of amphimictic females and the final production of infective juveniles. In in vitro studies, none of the contaminants supported the growth and development of axenic H. indica. Adequate precautions should be taken to maintain proper hygiene to eliminate such contaminants while culturing the Galleria and EPN for use in the biological control of insects.

In Vitro Effects of Streptomyces tyrosinase on the Egg and Adult Worm of Toxocara vitulorum

Background: Several species of streptomycetes, saprophytic bacteria found widely distributed in soil, water and plants, produce bioactive compounds such as intra and extracellular hydrolases including lytic enzymes which reflecting on their importance in the biological control of insects and parasites. This study assessed the in vitro effects of Streptomyces tyrosinase, produced from Streptomyces spp. isolated from Egyptian soil, on animal-parasitic nematode Toxocara vitulorum, in terms of egg development and adult worm’s cuticular structure, and as an alternative strategy to alleviate this infection. Methods: This study was conducted at the National Research Centre, Egypt in 2018. Five different concentrations of tyrosinase, ranged from 1%-30% were tested against the development of T. vitulorum eggs. The concentration induced the highest inhibitory activity was tested against adult T. vitulorum cuticle, which is essential for the protective and nutritive functions. The results were compared with those observed in the egg development and worm cuticle following incubation in Streptomyces protease (as a reference enzyme). Results: Compared to Streptomyces protease, higher inhibitory activity on T. vitulorum egg development and extreme cuticular alterations of the treated adult worms had been observed following 24 h exposure to Streptomyces tyrosinase. Once the cuticle had been damaged, the enzyme would be able to penetrate deeper into the internal tissues of the nematode and caused more widespread disruption. Conclusion: The current study could offer a promising bio-control agent, Streptomyces tyrosinase, against T. vitulorum alternative to the more expensive synthetic anthelmintics.

Isolates of Bacillus thuringiensis from Maranhão biomes with potential insecticidal action against Aedes aegypti larvae (Diptera, Culicidae)

Entomopathogenic agents are viable and effective options due to their selective action against insects but benign effects on humans and the environment. The most promising entomopathogens include subspecies of Bacillus thuringiensis (Bt), which are widely used for the biological control of insects, including mosquito vectors of human pathogens. The efficacy of B. thuringiensis toxicity has led to the search for new potentially toxic isolates in different regions of the world. Therefore, soil samples from the Amazon, Cerrado and Caatinga biomes of the state of Maranhão were evaluated for their potential larvicidal action against Aedes aegypti. The isolates with high toxicity to mosquito larvae, as detected by bioassays, were subjected to histological evaluation under a light microscope to identify the genes potentially responsible for the toxicity. Additionally, the toxic effects of these isolates on the intestinal epithelium were assessed. In the new B. thuringiensis isolates toxic to A. aegypti larvae, cry and cyt genes were amplified at different frequencies, with cry4, cyt1, cry32, cry10 and cry11 being the most frequent (33-55%) among those investigated. These genes encode specific proteins toxic to dipterans and may explain the severe morphological changes in the intestine of A. aegypti larvae caused by the toxins of the isolates.

Gnotobiotic Rearing and Controlled Infection with Gut Symbionts Improve Adult Fitness in Transgenic Diamondback Moth, Plutella xylostella

Mass insect rearing can have a range of applications, for example in biological control of insects. Since the performance of released biological control agents determines efficacy, the competitive fitness of insects post release is a key variable. Here, we tested whether inoculation with a gut symbiont, Enterobacter cloacae, and gnotobiotic rearing of larvae could improve insect growth and male competitive fitness of a transgenic diamondback moth, which has shown variation in fitness when reared in different insectaries. All larvae were readily infected with the focal symbiont. Under gnotobiotic rearing pupal weights were reduced and there was a marginal reduction in larval survival. However, gnotobiotic rearing substantially improved the fitness of transgenic males. In addition, in gnotobiotic conditions, inoculation with the gut symbiont increased pupal weights and male fitness, increasing the proportion of transgenic progeny from 20 to 30% relative to symbiont-free insects. Gnotobiotic conditions may improve the fitness of transgenic males by excluding microbial contaminants, while symbiont inoculation could further improve fitness by providing additional protection against infections, or by normalizing insect physiology. The simple innovation of incorporating antibiotic into diet, and inoculating insects with symbiotic bacteria that are resistant to that antibiotic, could provide a readily transferable tool for other insect rearing systems.